Method of drilling and operating a subsea well

ABSTRACT

A method of preparing and operating a subsea well ( 23 ), comprising the steps of: locating a drill-through subsea tree ( 24 ) on a subsea wellhead; sealingly connecting a high pressure drilling riser ( 22 ) between the tree and a drilling platform at the sea surface; mounting a blow out preventer ( 16 ) at the top of the riser ( 22 ); drilling the well through the drilling riser and the subsea tree; and establishing a production connection between the tree ( 24 ) and a production collection facility at the sea surface through a production riser ( 52 ) which is separate from the drilling riser ( 22 ).

This invention relates to a method of preparing and operating a subseawell, and to subsea well components for use in such a method. Theinvention is particularly intended for use in floating drilling andproduction system used in the recovery of offshore oil and gas reservesin deep water environments. ‘Deep water’ environments are usuallyconsidered as those where the operating depth is 800 metres or more.

A number of deep water reservoirs have been or are proposed to bedeveloped using floating vessels with vertically tensioned high pressuredrilling and production risers. This approach allows both the drillingBOP (blow out preventer) and production tree to be located on the vessel(often referred to as a ‘dry tree’ arrangement) providing access to theBOP and production tree reducing the duration of drilling and workoveroperations and cost. In this arrangement particular attention must bepaid to well control requirements since in the event of a riser failurethe well can be left in an unstable condition, resulting in anuncontrolled blow-out situation. To achieve acceptable reliability dualcasing risers are utilised to provide redundant pressure barriers.However, as the search for hydrocarbons extends to even greater waterdepths the complexity of these high pressure dual casing risers andparticularly their suspended weight becomes a significant cost driver.

According to the invention, there is provided a method of preparing andoperating a subsea well, the method comprising the steps of

-   -   locating a drill-through subsea tree on a subsea wellhead    -   sealingly connecting a high pressure drilling riser between the        tree and a drilling platform at the sea surface    -   mounting a blow out preventer at the top of the riser    -   drilling the well through the drilling riser and the subsea        tree; and    -   establishing a production connection between the tree and a        production collection facility at the sea surface through a        production riser which is separate from the drilling riser.

Use of this method offers the ability to efficiently drill, produce andworkover subsea wells in deep water by combining manifoldeddrill-through spool trees, a high pressure drilling riser with surfaceBOP and free standing offset production risers.

This can simplify the riser design, reduce the number of risers requiredand simplify the interface with a vessel or platform with the objectiveof reducing cost and improving safety.

In the invention, production risers can be run through the moonpool of amoored drilling vessel and a smaller vessel can be used to do the finalinstallation onto the riser foundation.

The pressure retaining drilling riser may comprise two concentric riserpipes, the inner of the pipes being a high pressure riser and the outerof the pipes being a low pressure riser. The low pressure riser can thenbe first connected between the tree and the drilling platform with a lowpressure blow out preventer mounted at the top of the riser. The wellcan then be drilled to a first depth, the low pressure blow outpreventer removed and the high pressure riser run into the low pressureriser. A high pressure blow out preventer is then mounted at the top ofthe high pressure riser, and the well is drilled to a second, greaterdepth.

A plurality of wells can be drilled adjacent to one another, and theproduction outflows from adjacent wells can be comingled in a manifoldat the seabed before being introduced to the production connection tothe sea surface.

The drilling platform and the production collection facility can beprovided on separate platforms/vessels at the sea surface.

The invention also provides a drill-through subsea tree adapted for usein the method set forth above. In particular, the tree may have anoutlet for connection to a production pipe, and means for providing adirect sealing connection to a pressure retaining riser so that drillingcan take place through the riser and through the subsea tree.

The invention includes a permanently moored floating drilling andproduction system for deep water comprising: a) manifolded drill throughsubsea trees located (directly) below the floating vessel; b) a highpressure drilling workover riser and c) a surface BOP configured withthe objective of reducing riser numbers and tension requirement andmaintaining vertical wellbore access.

The invention also includes a riser system for use in drilling andproducing a deepwater well from a permanently moored floating vesselcomprising: a high pressure vertically tensioned marine riser systemwith surface BOP for drilling operations extending downwardly from thesurface vessel and connected and sealed to a drill through subsea treethat is attached to a subsea wellhead located substantially below thesurface vessel.

The invention also includes a riser system for use in drilling andproducing a deepwater well from a permanently moored floating vesselcomprising dual string concentric pipe arrangement comprising: 1) anouter riser extending from the surface vessel connected and sealed to adrill through subsea tree that is connected onto a subsea well fordrilling an initial low pressure interval; 2) a retrievable inner riserextending from the surface downwardly to the subsea tree inside theouter riser and connected and sealed on the bore of the subsea tree orwellhead.

The inner string can be a casing.

On completion of a well, the riser system can be disconnected from thedrill through subsea tree, lifted slightly and moved across onto andconnected and sealed to another drill through subsea tree for drillingor intervention.

The invention also extends to a floating drilling and production vesselwith multiple drill through subsea trees located below the vessel withdrilling and workover conducted vertically using high pressure riserswith surface BOP.

This surface vessel can use free standing production risers to transfercommingled fluids from drill through subsea trees to the drilling andproduction vessel.

Free standing offset risers can be used to transfer commingled fluidsfrom subsea trees to an adjacent storage.

The offset risers can be initially connected to the drilling/productionfacility for early production and subsequently disconnected andreconnected to an adjacent storage facility. The risers can be installedthrough the moonpool or over the side of a permanently moored drillingand production vessel.

The production risers can be assembled using threaded connections.

The invention also extends to the installation of manifold structuresthrough the moonpool of a moored drilling and production vessel wherethe manifold is initially run in a vertical orientation in order to passthe moonpool and subsequently rotated horizontally after landing on theseabed or in midwater.

Still further, the invention extends to the installation of nearneutrally buoyant rigid flowline spools to which the manifold has beenassembled using threaded connections and initially run in the verticalorientation and subsequently rolled over to the horizontal orientationafter passing through the moonpool of the vessel.

Yet another feature of the invention is the use of a radial orientationkey in the bore of a high pressure drilling riser to locate and align atubing hanger landed in the bore of a drill through spool tree

In the event that a single string drilling riser is utilised a shear rammodule may be used at the base of the riser to isolate the well in theevent of a riser failure. The shear ram module will connect to thesubsea tree mandrel via a remote connector and will have at its top enda mandrel onto which the drilling riser is connected. In the unlikelyevent that the shear ram is actuated to close in the well, the drillingriser is then retrieved and repaired prior to reinstallation on theshut-in well.

The invention thus relates to an offshore production system for deep andultra deep water developments that allows drilling, production andworkover of subsea wells.

There are four main elements used in the production system, ie

-   -   Moored floating production unit    -   Drill through subsea trees with compact manifold    -   High pressure drilling riser    -   Free standing offset production risers

The floating production unit may take a number of different formsincluding barge, ship, semi-submersible, TLP (tension leg platform) orSpar. However, in its simplest form it consists of a flat bottom bargeconstructed from either steel or concrete. The barge provides drilling,production, storage and accommodation facilities with the drillingfacilities being located near the centre of the vessel where vesselmotions are smallest. A drilling derrick is located directly above acentral moonpool that facilitates installation of a high pressuredrilling riser.

A vertically tensioned high pressure drilling riser is proposed, similarto that used on existing deepwater developments. The riser pipe isconstructed from steel tubulars, connected by flanged couplings. Theriser is rated to the maximum reservoir pressure and a surface BOP isused to control the flow of drilling fluids and returns in and out ofthe well bore. The drilling riser may be either a single string or adual string concentric arrangement. If a single string riser is used ariser base shear ram module may also be used immediately above the tree.The BOP is located in the moonpool directly below the derrick.

During drilling operations control of the subsea tree and riser baseshear ram module is provided via a control umbilical run on the outsidediameter of the drilling riser. Following connection of the drillingriser to the tree control of the tree functions is provided and theproduction control is isolated.

Subsea trees and manifolds are located on the seabed below theproduction vessel. The trees are ‘spool’ or ‘drill through’ designallowing full bore access to the well onto which they are connected. Thetrees can be installed on to the subsea wellhead on the bottom end ofthe drilling riser with subsequent drilling activities conducteddirectly through the tree.

Well intervention and light workover operations can be completed througha small bore high pressure riser typically 8⅝ inch diameter. The singlestring riser is run down and attached to the upper mandrel of the treein the same way as the drilling riser. A similar surface BOP, but ofsmaller internal diameter is used at the surface. Such a lightweightriser system allows access into the production tubing. For additionalsafety an isolation valve may be included at the base of the risercapable of shearing wireline, slickline and coiled tube.

A number of trees (typically five) are arranged on each of fourmanifolds, which commingles the flow from each tree and directs the flowto an adjacent production riser base. A number of such tree andmanifolds may be used, typically providing a total of twenty subseatrees.

Each manifold is connected to an adjacent offset production riser viaspools that provide production, annulus access and control functions.The offset riser consists of near vertical steel pipes connected bythreaded couplings. The risers are vertically supported by near surfaceaircans which maintain tension in the riser sufficient to withstandenvironmental and operational loads. At the top of the riser a flexiblepipe jumper is used to connect between the riser and the productionvessel. The production riser may be single string for service ie. waterinjection or concentric dual string for production where the outerannulus may be used for insulation of gas injection/lift.

It will be apparent to a person skilled in this technology that thisarrangement greatly reduces the number of production risers required forsuch a development from approximately twenty to five, since the wellsare manifolded subsea. This reduces riser steel weight, tensioningrequirements and wellbay size. Furthermore the arrangement facilitatessubsea wellbore isolation at the subsea wellhead improving safety,reliability of such a system and simplifies the wellbay and moonpoollayout. Most importantly these benefits are provided without the loss ofvertical wellbore access for drilling and workover and with the abilityto use a high pressure drilling riser and surface BOP.

The invention will now be described in more detail by way of exampleonly with reference to the accompanying drawings in which:

FIG. 1 is a general view showing the method of the invention in use;

FIG. 1 a is a view similar to that of FIG. 1 but showing an alternativearrangement;

FIG. 2 is a cross-section through a subsea tree and manifold for use inthe invention;

FIG. 3 is a plan view of a manifold;

FIG. 4 is a side view of the manifold of FIG. 3;

FIG. 5 shows a detail of the offset riser upper assembly; and

FIG. 6 shows a detail of the offset riser base arrangement

Drilling takes place from a vessel 10 which consists of a steel orconcrete barge with a central moonpool 12. The vessel is permanentlyspread moored for the duration of the field life or alternatively may beturret moored. Typical dimension of the barge are 175 m long, 60 m widthand a depth of 15 m. It will however be clear to the skilled man thatthese dimensions may be varied according to the requirements of eachparticular development. The main function of the barge is to providedrilling and workover for subsea wells that are located directly belowthe vessel. However, the barge may also provide other functions such aspersonnel accommodation, process and storage.

Drilling and workover of a well 23 takes place through a high pressureriser 22. Production, ie the bringing of oil or gas form the well 23 tothe surface, takes place through an offset production riser 52.

Drilling and workover is conducted through the central moonpool 12 whichis typically 10 m×15 m plan area, allowing installation of manifolds,trees, drilling riser and offset production risers. The drillingfacilities consist of a conventional derrick 14 and mud and pipehandling facilities. The drilling facilities are modular and can beskidded onto the barge 10 during barge construction and possibly removedat the end of the drilling phase. The arrangement uses a surface BOP 16,which is located within the moonpool immediately below the drill floor.Sufficient vertical space is provided to accommodate stroking of the BOPin the worst anticipated storm condition without impact with the hullstructure.

Flexible jumpers 18 from the offset production risers 50,52 areconnected to the barge via porches 20 located on the side of the barge.The porches are located away from the drilling area to provide goodseparation between drilling and production facilities for safetyreasons.

The drilling riser 22 extends below the barge 10 to the wells on theseabed and is rated to resist the maximum shutin pressure of thereservoir. Isolation is achieved by the use of a surface BOP. The risermay be either single string or dual string, depending on particularreservoir parameters. A single string riser could have a diameter ofapproximately 24 inches and a dual string (concentric) riser could havepipe dimensions typically 22 inches diameter for the outer pipe and 13⅜inch diameter for the internal liner.

The drilling riser 22 is run through the moonpool of the productionvessel and is assembled from a series of riser joints (22 a, 22 b, 22 c,. . . ) using mechanical connections. At the bottom end the riser pipeis attached to the upper mandrel of a production tree 24 via a hydrauliccollet connector. The tree will be described in more detail withreference to FIG. 2.

A tapered stress joint is used between the tree connector and the firststandard riser joint to accommodate local stresses resulting fromenvironmental loading and vessel offset. The tapered stress joint is apipe section with an increasing wall thickness along its length toresist bending loads. The taper joint may be manufactured from steel ortitanium if lower wellhead loads are required. The bore of the taperjoint incorporates an orientation pin or similar device to alloworientation of the tubing hanger and tubing hanger running tool. The pinis hydraulically extended into the bore of the taper joint and impingeson a helical profile provided on the tubing hanger running tool.

In use, the riser 22 is lowered through the moonpool 12 and connected tothe subsea tree 24 at the top of a well 23. The tree 24 is preinstalledon a subsea manifold 26. After connection to the subsea tree thedrilling riser is tensioned within the moonpool of the barge using ahydro-pneumatic system. The uppermost joint of the riser string ismachined with a profile to accept the BOP 16 and a conventional diverter(not shown) is located below the drill floor.

In an alternative embodiment, the subsea tree can be installed on thebottom of the drilling riser and can be lowered to the seabed with theriser.

Where a dual string riser 22 is being used, after drilling the top holesection, an internal smaller diameter casing is run inside the outerpipe to provide a high pressure liner through which the remaining bottomhole section is drilled. The liner is assembled from threaded casing andis latched into a profile inside the bore of the subsea wellhead 24.This requires the liner to be installed through the taper joint and boreof the subsea tree 36. Once the liner is latched and sealed to the boreof the wellhead it is pretensioned at the surface against the outer 22inch pipe using a bowl and slip assembly. A high pressure surface BOP isattached to the upper end of the liner for drilling the higher pressurebottom hole section. Alternatively the internal liner can be latched andsealed in the bore of the subsea tree.

Following drilling to total depth the well is completed by installationof production tubing and tubing hangers. This is achieved in the case ofthe dual string drilling riser by removal of the inner liner to providefull bore access through the drilling riser to allow passage of thetubing hanger, which is landed in the body of the tree.

After drilling and completing on one well the drilling riser isdisconnected from the tree, lifted slightly and then jumped across tothe next well to be drilled or requiring intervention, without retrievalto the surface. The subsea tree 24 (FIG. 2) is a ‘drill through’ design.

FIG. 2 shows the manifold 26, a wellhead 30 and a subsea tree 24 mountedon the wellhead. This Figure shows a dual string riser with an outer lowpressure drilling riser 34 and an inner high pressure riser 36. The highpressure riser has to be sealed to the wellhead, and FIG. 2 shows twopossible ways in which this sealing can be completed. On the right handside of the centre axis, the riser 36 is shown sealed by seals 38directly in a bore in the well head 30. On the left hand side of thecentreline, the riser is shown sealed indirectly to the wellhead byseals 40 in the tree 24. The tree is then itself sealed to the wellhead30 by further seals at 42.

The well casings are hung from hangers 70 in the wellhead 30. The tree24 has a downwardly flared connector collar 72 which locates over thetop of the tree. The tree has production valves 74 and a productionchoke 76. At the upper end of the tree, there is a re-entry funnel 78 tofacilitate re-entry to the well.

On completion of the well the tubing hanger is landed in the bore of thetree spool with horizontal wing outlets. When the tubing hanger is notinstalled, full bore access is provided through the tree for access intothe lower wellbore. This allows the high pressure liner 36 to be runthrough the tree and landed and locked inside the wellhead (see 38 inFIG. 2).

The tree 24 will have a machined profile on the bore of the spool. Thiswill be used to latch and seal the internal tieback sleeve (if a dualconcentric design is used).

A template 32 (see FIG. 3) is used to determine the positions of thewells. The template is designed so that it can be installed through themoonpool 12 of the barge 10. Each template has locations 44 throughwhich wells will be drilled. In the example shown there are locationsfor up to five wells. The locations are arranged in a row such that thetemplate is long and thin and can pass through the moonpool vertically.The template incorporates temporary mudmats for stability prior todrilling and incorporates piping and valving for serving each welldrilled through the template, and a manifold 26 to which the piping isconnected so that the manifold can commingle production flow and,distribute control functions to individual trees located in thelocations 44.

An alternative arrangement is to locate the wells off the template andconnect them to the template using jumpers. This allows the size of thetemplate structure to be reduced.

An umbilical 66 (FIG. 1) can be used to control some of the treefunctions. The umbilical will be run down the outside diameter of thedrilling riser and will terminate at a stab plate 68 adjacent to thebase riser connector. The stab plate 68 mates with a similar stab plate70 on the tree.

The templates are lowered through the moonpool vertically on drill pipeand rotated to the horizontal after passing the keel or on the seabed.The manifold can be installed complete with jumper spools 50 that, inuse, connect the manifold to the base of the offset production riser 52.These jumper spools may be 200–300 m long. The spools are assembled inthe moonpool using threaded connections and are neutrally buoyant inwater due to being air filled and coated with a thick and lightweightthermal insulation material.

The manifolds are lowered and positioned on the seabed so that the endof the jumper spools connects with or lands close to preinstalledfoundation piles 51 onto which the offset risers are attached. Multiplemanifold units can be used, depending on the total number of wellsrequired. The manifolds are positioned on the seabed to allow goodaccess from the barge, to protect the wellheads from dropped objects andto allow the required distribution of offset risers 52 around theperimeter of the barge.

The offset risers 52 consist of a pipe in pipe configuration. Thecentral pipe diameter is sized for the main flow path and the annulusbetween the central and outer pipe is filled with air and a vapour phasecorrosion inhibitor which, together with the buoyancy material aroundthe outer pipe, provides thermal insulation to the production pipe.Alternatively the annulus can be used for gas lift or gas injection andwill then be filled with pressurised hydrocarbon gas. Preheating the gasprior to injection into the riser provides an effective means of heatingthe central pipe to maintain product arrival temperatures.

The outside surface of the outer pipe 52 is coated with a corrosionprotection material such as fusion-bonded epoxy or thermally sprayedaluminium. Buoyancy material is attached to the large diameter pipe,which is sized such that the pipe section is near neutrally buoyant inwater in the production mode.

The production riser 52 is offset from the production vessel, and istensioned using aircans 54 connected to the top of the riser 52. Theaircans are attached to the riser by an articulation 56 that allows theaircan to rotate independently to the riser.

Beneath the articulation a gooseneck assembly 58 (FIG. 5) is located toprovide a fluid outlet flow path to jumpers 18. The jumpers connect theriser 52 to the vessel 10 and are assembled from flexible pipemanufactured from steel reinforced thermoplastic materials. The jumpersare configured in free hanging catenaries and connect to porches 20located on the perimeter of the barge. Alternatively the jumpers can beconnected directly to an adjacent storage facility 80 and not to thedrilling barge. This is shown in FIG. 1 a. A third option is to connectthe jumpers to the drilling barge 10 for early production and at a laterdate transfer the jumpers over to the adjacent storage facility 80 forthe remaining life of field.

Below the gooseneck is a spool 60 machined with an internal profile usedto suspend and pre-tension the internal pipes of the riser 52 inside theouter carrier.

The spool interfaces with the gooseneck assembly providing flow pathsand communication with the gooseneck. The design of the spool is similarto that used for wellhead tubing hangers wherein a hanger, complete withseals and lock down mechanisms is located within an outer wellhead orbowl.

At the base of the production riser 52 (FIG. 6), the riser is connectedto a mandrel profile fabricated onto the upper end of a pile that may bedrilled and grouted or jetted. A flowbend 62 with outboard hub 63 isincorporated at the bottom of the riser string. The hub 63 allowsconnection of the jumper spool 50 (which itself connects to the subseatrees 24) via a vertically installed spool 64.

The system described here allows subsea wells to be drilled and thenbrought into production in an efficient and simple manner.

It is an advantage of the invention that the production risers can berun through the moonpool of the moored drilling vessel and a smallervessel can be used to do the final installation onto the riserfoundation.

1. A method of preparing and operating a subsea well, the methodcomprising the steps of: locating a plurality of drill-through subseatrees on a respective plurality of subsea wellheads, each of theplurality of drill-through subsea trees being connected by a manifold;sealingly connecting a high pressure drilling riser between one of theplurality of trees and a drilling platform at the sea surface; mountinga blow out preventer at the top of the riser; drilling the well throughthe drilling riser and the subsea tree; and establishing a productionconnection between the tree and the manifold through a production riserwhich is separated from the drilling riser; wherein the high pressuredrilling riser comprises two concentric riser pipes, the inner of thepipes being a high pressure riser and the outer of the pipes being a lowpressure riser; and wherein the low pressure riser is first connectedbetween the tree and the drilling platform with a low pressure blow outpreventer mounted at the top of the riser, the well is drilled to afirst depth, the low pressure blow out preventer is removed, the highpressure riser is run into the low pressure riser, a high pressure blowout preventer is mounted at the top of the high pressure riser, and thewell is drilled to a second, greater depth.
 2. A method of preparing andoperating a subsea well, the method comprising the steps of: locating aplurality of drill-through subsea trees on a respective plurality ofsubsea wellheads, each of the plurality of drill-through subsea treesbeing connected by a manifold; sealingly connecting a high pressuredrilling riser between one of the plurality of trees and a drillingplatform at the sea surface; mounting a blow out preventer at the top ofthe riser; drilling the well through the drilling riser and the subseatree; and establishing a production connection between the tree and themanifold through a production riser which is separated from the drillingriser; wherein the drilling platform and a production collectionfacility are provided on separate platforms at the sea surface.
 3. Amethod as claimed in claim 2, wherein the one or more production risersare initially connected to the drilling platform for early productionand are subsequently disconnected and reconnected to the productioncollection platform.
 4. A method of preparing and operating a subseawell, the method comprising the steps of: locating a plurality ofdrill-through subsea trees on a respective plurality of subseawellheads, each of the plurality of drill-through subsea trees beingconnected by a manifold; sealingly connecting a high pressure drillingriser between one of the plurality of trees and a drilling platform atthe sea surface; mounting a blow out preventer at the top of the riser;drilling the well through the drilling riser and the subsea tree;establishing a production connection between the tree and the manifoldthrough a production riser which is separated from the drilling riser;and providing one or more manifolds to connect the plurality ofdrill-through subsea trees; and wherein a single production riser isassociated with each manifold; wherein the or each manifolds areinstalled through a moonpool of a moored, floating vessel and areinitially run in a vertical orientation in order to pass the moonpooland are subsequently rotated horizontally after landing on the seabed orin midwater.
 5. A floating drilling and production system for deep watercomprising a floating vessel, a plurality of drill-through subsea treeslocated below the floating vessel, the plurality of trees beingconnected to a subsea manifold, a high pressure drilling workover riserextending between one of the plurality of trees and the vessel, ablow-out preventer located on the vessel at the top of the riser, and aproduction riser extending from the manifold to near the water surface,the production riser being separated from the drilling riser; whereinthe drilling riser comprises means by which it can be selectivelycoupled to one of the trees and can be moved from one tree to another.6. A floating drilling and production system for deep water comprising afloating vessel, a plurality of drill-through subsea trees located belowthe floating vessel, the plurality of trees being connected to a subseamanifold, a high pressure drilling workover riser extending between oneof the plurality of trees and the vessel, a blow-out preventer locatedon the vessel at the top of the riser, and a production riser extendingfrom the manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein there are a plurality ofmanifolds, and each manifold has a production riser.
 7. A floatingdrilling and production system for deep water comprising a floatingvessel, a plurality of drill-through subsea trees located below thefloating vessel, the plurality of trees being connected to a subseamanifold, a high pressure drilling workover riser extending between oneof the plurality of trees and the vessel, a blow-out preventer locatedon the vessel at the top of the riser, and a production riser extendingfrom the manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein the drilling riser comprisesa dual string concentric pipe arrangement with an outer riser extendingfrom the floating vessel connected and sealed to a drill through subseatree connected onto a subsea well, for drilling an initial low pressureinterval, and a retrievable inner riser extending from the surface tothe subsea tree inside the outer riser and connected and sealed on thebore of the subsea tree or wellhead.
 8. A floating drilling andproduction system as claimed in claim 7, wherein the inner string is acasing.
 9. A floating drilling and production system for deep watercomprising a floating vessel, a plurality of drill-through subsea treeslocated below the floating vessel, the plurality of trees beingconnected to a subsea manifold, a high pressure drilling workover riserextending between one of the plurality of trees and the vessel, ablow-out preventer located on the vessel at the top of the riser, and aproduction riser extending from the manifold to near the water surface,the production riser being separated from the drilling riser; wherein acontrol umbilical is run on the outside diameter of the drilling riserto provide control of the subsea tree.
 10. A floating drilling andproduction system for deep water comprising a floating vessel, aplurality of drill-through subsea trees located below the floatingvessel, the plurality of trees being connected to a subsea manifold, ahigh pressure drilling workover riser extending between one of theplurality of trees and the vessel, a blow-out preventer located on thevessel at the top of the riser, and a production riser extending fromthe manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein the manifold is connected toan adjacent offset production riser via spools that provide production,annulus access and control functions.
 11. A floating drilling andproduction system as claimed in claim 10, wherein the or each offsetriser consists of near vertical steel pipes connected by threadedcouplings.
 12. A floating drilling and production system for deep watercomprising a floating vessel, a plurality of drill-through subsea treeslocated below the floating vessel, the plurality of trees beingconnected to a subsea manifold, a high pressure drilling workover riserextending between one of the plurality of trees and the vessel, ablow-out preventer located on the vessel at the top of the riser, and aproduction riser extending from the manifold to near the water surface,the production riser being separated from the drilling riser; whereinthe system further comprises a production vessel, the production riserbeing connected to the production vessel.
 13. A floating drilling andproduction system for deep water comprising a floating vessel, aplurality of drill-through subsea trees located below the floatingvessel, the plurality of trees being connected to a subsea manifold, ahigh pressure drilling workover riser extending between one of theplurality of trees and the vessel, a blow-out preventer located on thevessel at the top of the riser, and a production riser extending fromthe manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein the production riser isvertically supported by near surface aircans which maintain tension inthe riser sufficient to withstand environmental and operational loads.14. A floating drilling and production system as claimed in claim 13,wherein a flexible pipe jumper is used at the top of the productionriser to connect between the riser and the production vessel.
 15. Afloating drilling and production system for deep water comprising afloating vessel, a plurality of drill-through subsea trees located belowthe floating vessel, the plurality of trees being connected to a subseamanifold, a high pressure drilling workover riser extending between oneof the plurality of trees and the vessel, a blow-out preventer locatedon the vessel at the top of the riser, and a production riser extendingfrom the manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein the or each production riseris concentric dual string for production and the outer annulus may beused for insulation of gas injection/lift.
 16. A floating drilling andproduction system for deep water comprising a floating vessel, aplurality of drill-through subsea trees located below the floatingvessel, the plurality of trees being connected to a subsea manifold, ahigh pressure drilling workover riser extending between one of theplurality of trees and the vessel, a blow-out preventer located on thevessel at the top of the riser, and a production riser extending fromthe manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein the or each manifold has nearneutrally buoyant rigid flowline spools.
 17. A floating drilling andproduction system for deep water comprising a floating vessel, aplurality of drill-through subsea trees located below the floatingvessel, the plurality of trees being connected to a subsea manifold, ahigh pressure drilling workover riser extending between one of theplurality of trees and the vessel, a blow-out preventer located on thevessel at the top of the riser, and a production riser extending fromthe manifold to near the water surface, the production riser beingseparated from the drilling riser; wherein a radial orientation key isprovided in the bore of the high pressure drilling riser to locate andalign a tubing hanger landed in the bore of a drill-through spool tree.18. A floating drilling and production system for deep water comprisinga floating vessel, a plurality of drill-through subsea trees locatedbelow the floating vessel, the plurality of trees being connected to asubsea manifold, a high pressure drilling workover riser extendingbetween one of the plurality of trees and the vessel, a blow-outpreventer located on the vessel at the top of the riser, and aproduction riser extending from the manifold to near the water surface,the production riser being separated from the drilling riser wherein thedrilling riser is a single string drilling riser, and a shear ram moduleis provided at the base of the riser to isolate the well in the event ofa riser failure and wherein the shear ram module connects to the subseatree mandrel via a remote connector and has at its top end a mandrelonto which the drilling riser is connected.